A metal-organic framework (MOF) material was developed that exhibits a selective, fully reversible, and repeatable capability to remove nitrogen dioxide gas from the atmosphere in ambient conditions. The material denoted as MFM-300 (Al) exhibited the first reversible, selective capture of nitrogen dioxide at ambient pressures and temperatures — at low concentrations — in the presence of moisture, sulfur dioxide, and carbon dioxide. Despite the highly reactive nature of nitrogen dioxide, the MFM-300(Al) material proved extremely robust, demonstrating the capability to be fully regenerated, or degassed, multiple times without loss of crystallinity or porosity. Other different porous materials often showed degraded performance in subsequent cycles by the nitrogen dioxide, or that the regeneration process was too difficult and costly.
As part of the research, neutron scattering techniques were used to confirm and precisely characterize how MFM-300(Al) captures nitrogen dioxide molecules. Neutrons can easily penetrate dense materials, and they are sensitive to lighter elements, such as the hydrogen atoms inside MFM, which enabled researchers to observe how the nitrogen dioxide molecules are confined within the nano-size pores. The ability to directly observe how and where MFM-300(Al) traps nitrogen dioxide is helping the researchers validate a computer model of the MOF gas separation process, which could help identify how to produce and tailor other materials to capture a variety of different gases. Computer modeling and simulation played critical roles in interpreting the neutron scattering data by helping to connect subtle changes in the vibrational spectra to interactions between the MFM-300 and trapped molecules.
Capturing greenhouse and toxic gases from the atmosphere has long been a challenge because of their relatively low concentrations and the presence of moisture in the air, which can negatively affect separating targeted gas molecules from other gases. Another challenge has been finding a practical way to release a captured gas for long-term sequestration, such as in underground depleted oil reservoirs or saline-filled rock formations. MOFs offer solutions to many of these challenges.